1. Field of the Invention
The present invention relates to an image pickup apparatus capable of selecting a focus detection area (focus detection point).
2. Description of the Related Art
In many image pickup apparatus such as a single-lens reflex camera, the autofocus by a TTL phase difference detection method can be performed. In the TTL phase difference detection method, a pair of object images are formed by dividing luminous flux from an area corresponding to a focus detection point set in an image taking region of an image pickup optical system, and a pair of image signals are obtained by performing a photoelectric conversion of the pair of object images by a line sensor (a photo electric conversion element array). Then, a defocus amount of the image pickup optical system is obtained based on a phase difference of the pair of image signals and a focus lens is moved by a movement amount obtained based on the defocus amount to obtain an in-focus state.
In many cases, a plurality of focus detection points (also referred to as focus detection areas) are provided in the image taking region. In this case, the focus detection is performed by using the focus detection point automatically selected by the image pickup apparatus or arbitrarily selected by a user out of the plurality of focus detection points.
Furthermore, in order to increase the flexibility of selecting a composition at the time of taking an image, it is preferable that as many focus detection points as possible are densely arranged in the image taking region. In order to increase the number of the focus detection points, for example as illustrated in FIG. 10A, there is a method in which one long line sensor is divided into a plurality of (five) photoelectric conversion element blocks Line1 to Line5 to form one focus detection point by each photoelectric conversion element block.
However, in the method of dividing the line sensor illustrated in FIG. 10A, the length of the photoelectric conversion element block per one focus detection point is short, and therefore a range of a detectable defocus amount is narrow.
On the other hand, as illustrated in FIG. 10B, if the photoelectric conversion element blocks are set so as to be overlapped with each other, the length of the photoelectric conversion element block per one focus detection point can be enlarged. In this case, a charge accumulation control of the long line sensor is performed at once and an image signal section corresponding to one of the photoelectric conversion blocks is extracted from an image signal read from the line sensor to perform focus detection at one focus detection point using the image signal section.
In Japanese Patent Laid-open No. H08-15603, an image pickup apparatus which performs focus detection by a method as illustrated in FIG. 10B is proposed. In the image pickup apparatus, whether near and far objects are generated at one focus detection point is determined. If it is determined that the near and far objects are generated at the one focus detection point, the method of dividing the photoelectric conversion element blocks is changed, i.e. the positions of the photoelectric conversion element blocks are displaced, to perform the focus detection again.
As disclosed in Japanese Patent Laid-open No. H08-15603, the positions of the photoelectric conversion element blocks are displaced to be able to variously change a position of a focus detection point.
However, even if the positions of photoelectric conversion elements are displaced, an optimal charge accumulation control cannot be performed for each photoelectric conversion element block if there is an overlapped portion between the plurality of photoelectric conversion element blocks. If the charge accumulation control is not optimized for each photoelectric conversion element block, reliable focus detection cannot be performed because a good image signal cannot be obtained.
For example, as illustrated in FIG. 11, a case where a bright object image is formed on a photoelectric conversion element block Line1 and also a dark object image is formed on a photoelectric conversion element block Line2 will be described. In this case, when the optimal charge accumulation control is performed for the photoelectric conversion element block Line1, an image signal having sufficient amplitude cannot be obtained from the photoelectric conversion element block Line2. On the other hand, when the optimal charge accumulation control is performed for the photoelectric conversion element block Line2, an image signal of the photoelectric conversion element block Line1 is saturated. In both cases, a good image signal cannot be obtained.
In order to resolve such problems, as illustrated in FIG. 12, a method of switching and using a division pattern A by which one long line sensor is divided into three photoelectric conversion element blocks and a division pattern B by which the sensor is divided into two electric conversion element blocks is considered. According to the method, an optimal charge accumulation control can be performed for each photoelectric conversion element block because five focus detection points can be formed and there is no overlapped region between the photoelectric conversion element blocks.
However, in this method, a plurality of charge accumulation operations, i.e. charge accumulation operations using the division pattern A and the division pattern B are necessary. Therefore, the time required for the AF increases.